1. Field of the Invention
The present invention relates to the field of power converters of low-voltage switch-mode power supply type. The present invention applies, more specifically, to non-isolated power supplies, that is, power supplies having a common node with the A.C. supply network, for example, the mains at 220 or 110 volts. It should however be noted that the present invention applies, more generally, to any switch-mode power supply system adapted to providing a D.C. voltage to a load.
2. Discussion of the Related Art
A switch-mode power supply circuit to which the present invention relates always includes an inductive element in series with a switch, to charge a capacitor across which the output voltage is sampled, a switch control circuit to regulate the output voltage by modifying the switch conduction periods and a free wheel diode of the inductive element. A switch-mode power supply circuit receives a D.C. voltage, generally, a rectified A.C. voltage.
FIG. 1 shows a conventional example of a switch-mode power supply of the type to which the present invention applies. In the example of FIG. 1, two input terminals P, N, receive an A.C. voltage Vac, for example, the mains voltage. Voltage Vac undergoes a rectifying, for example, a single-halfwave rectifying by means of a diode Dr, the anode of which is connected to terminal P and the cathode K of which provides a rectified voltage Vr. Voltage Vr is generally smoothed by means of a capacitor C1 connected between terminals K and N. Terminals K and N form the input terminals of the actual switch-mode power supply.
The circuit of FIG. 1 shows an assembly known as a "non-isolated voltage step-down converter", which is characterized by the fact that switch 1 (for example, an N-channel MOS transistor) is seriesconnected with an inductance Li and a capacitor Ci between terminals K and N of application of the rectified voltage. This converter operates in switched mode at a non-audible high frequency (generally over 20 kHz). Switch 1 is controlled by a circuit 2 which determines its on and off periods. A free wheel diode Di is connected in parallel with the series association of inductance Li and capacitor Ci, the cathode of diode Di being connected to the midpoint 3 of the series association of switch 1 with inductance Li.
The terminals of capacitor Ci form two output terminals Sp, Sn of the converter, intended for supplying a load (not shown) with a voltage Vout. Control circuit 2 of switch 1 is formed of a circuit of generation of a control gate signal for transistor 1 by pulse-width modulation. Circuit 2 is associated with a local supply assembly formed, in this example, of a power storage capacitor C1, across which circuit 2 is supplied. A first positive terminal PL of capacitor C1 is connected, via a resistor Rp, to terminal K providing rectified voltage Vr. The negative terminal NL of capacitor C1 is connected to midpoint 3 of the series association of transistor 1 with inductance Li, and forms the reference supply potential of circuit 2. A zener diode may be mounted in parallel with capacitor C1 to limit the value of the local supply voltage between two respectively positive and reference lines PL and NL, corresponding to the terminals of capacitor C1.
Generation and pulse-width modulation circuit 2 is, for example, an integrated circuit sold by STMicroelectronics under trade name UCC3824. This circuit includes two supply terminals Vdd, Vss, respectively connected to lines PL and NL, an output terminal s connected, possibly via a switching rate control resistor R1, to the gate of transistor 1, and three parametering input terminals OSC, Vfb, and COMP. Terminal OSC is intended for adjusting the frequency of an oscillator contained by circuit 2 to generate the control pulses. This frequency is conditioned by the respective values of a resistor R2 and of a capacitor C2 associated in series between lines PL and NL, the midpoint 4 of their series association being connected to terminal OSC. Terminal Vfb is intended for receiving a regulation order to have the width of the control pulses controlled by output voltage Vout, to maintain said voltage to the desired value. Terminal COMP is connected, via a resistor R3 in series with a capacitor C3, to line NL. Terminal COMP forms a compensation input terminal to stabilize the regulation loop.
The signal received by terminal Vfb of circuit 2 generally originates from an error circuit 5, or a circuit for measuring the drift of voltage Vout with respect to the desired value. Circuit 5 is generally formed of a comparator 6 (for example, an operational amplifier), a first (non-inverting) input terminal of which receives a reference voltage Vref, determined by a zener diode DZ, and a second (inverting) input terminal of which is connected to midpoint 7 of a series association of two resistors R4, R5, connected across capacitor Ci.
In the example shown in FIG. 1, the output of switch 6 is connected to the cathode of the photodiode 8 of an optocoupler 9. The anode of photodiode 8 is connected, via a resistor R6, to positive terminal Sp providing output voltage Vout. Operational amplifier 6 is generally associated with a feedback loop, for example, formed of a capacitor C7 in series with a resistor R7 between its output and its inverting input. Phototransistor 10 of optocoupler 9 is connected, by its collector, to positive local supply line PL of circuit 2 and, by its emitter, to terminal Vfb of circuit 2. A resistor R8 connects terminal Vfb to line NL, and forms a current-to-voltage converter at the input Vfb of circuit 2.
To reverse the control, a bipolar transistor (not shown) may be connected between the cathode of photodiode 8 and reference line Sn, the base of the transistor being connected to the output of comparator 6.
When the voltage sampled at node 7 exceeds the voltage threshold determined by zener diode DZ, photodiode 8 is crossed by a current and the potential of terminal Vfb is thus switched to a high level by the turning-on of phototransistor 10. Conversely, when phototransistor 10 is off, the potential of signal Vfb is low.
The respective sizings of resistive bridge R4, R5, and of diode DZ enable determining the regulation value of voltage Vout issued by capacitor Ci. For example, an operational amplifier 6, associated with a voltage reference Vref of 2.5 volts, is available in the form of an integrated circuit error amplifier, sold by STMicroelectronics under trade name TL431.
It should be noted that, according to the value of inductance Li, the current flowing therethrough is direct or not. It should also be noted that, for a non-isolated power supply, capacitors Ci and C1 have a common terminal. Further, if circuits of switching on the high rectified voltage present across capacitor C1 are desired to be made downstream, this common terminal is the negative terminal (cold point).
A first disadvantage of conventional switch-mode power supply-type conventional converters is the compulsory use of an isolating element formed, either of an optocoupler as illustrated in FIG. 1, or of a transformer. Indeed, integrated circuit 2 of control of switch 1 is referenced on source 3 of transistor 1 whereas the measurement performed by circuit 5 is referenced on negative terminal Sn of capacitor Ci. These different references make a direct connection between measurement circuit 5 and control circuit 2 impossible.
Another disadvantage is that the supply of control circuit 2 requires a resistor (Rp), which causes a high dissipation in the circuit. Indeed, this resistor must enable the supply of the pulse-width modulation circuit which requires, generally, a current of several tens of milliamperes, and must absorb the power due to the large potential difference present between terminals K and PL, in the middle of the halfwaves of voltage Vr if said voltage comes from the mains.
Another disadvantage of a conventional circuit such as shown in FIG. 1 is that the control block generally includes, when made in the form of an integrated circuit, its own error amplifier. Accordingly, circuit 5 is redundant but is necessary due to the presence of the isolating block (optocoupler 9).
FIG. 2 partially shows a second conventional example of a switch-mode power supply circuit. The example of FIG. 2 is known as a non-isolated voltage step-down/step-up converter and characterizes by the fact that capacitor Ci is no longer in series with inductance Li and the switch between the two terminals of application of rectified voltage Vr, the inductance still being in parallel with a series association of the free wheel diode Di with capacitor Ci. This arrangement enables operating the converter in voltage step-up mode with respect to the input voltage.
For clarity, FIG. 2 only shows those elements of the converter which differ from the assembly of FIG. 1. Thus, control circuit 2 and switch 1 as well as their associated parametering components have not been shown. It should however be noted that, in the assembly of FIG. 2, positive output terminal Sp corresponds to phase P of the A.C. power supply, the rectifying diode (Dr, FIG. 1) having its anode connected to the neutral of the A.C. power supply.
Measurement circuit 5' of a step-down/step-up converter such as shown in FIG. 2 is, for example, formed of an assembly based on a comparator 6 as in FIG. 1. However, the output of comparator 6 now is connected, via resistor R6, to the anode of photodiode 8 of optocoupler 9, the cathode of which is connected to reference terminal Sn of output voltage Vout. Zener diode DZ is connected between the non-inverting terminal of amplifier 6 and positive output terminal Sp, its anode being connected to the non-inverting input. In the example of FIG. 2, the feedback loop of amplifier 6 has been simplified to only include a capacitor C7. The emitter of phototransistor 10 is connected to terminal Vfb of circuit 2 (FIG. 1) and its collector is connected to line PL.
A step-down/step-up converter such as illustrated in FIG. 2 suffers from the same disadvantages as those described in relation with the converter of FIG. 1.
In some applications, N-channel MOS transistor 1 can be integrated with its electronic control circuit on a same chip. An example of such an integrated circuit is a circuit sold by STMicroelectronics, known under trade name VIPER.
FIG. 3 shows an example of a conventional diagram implementing component 11. The representation of FIG. 3 is partial in that it only shows the control portion of switch 1', integrated to circuit 11. The rest of the assembly is similar, either to the assembly of FIG. 1, or to the assembly of FIG. 2 according to the type of converter made.
In FIG. 3, VIPER component 11 has been symbolized by its integrated switch 1', controlled in pulse-width modulation by an amplifier 12 receiving, as an input, an oscillation order provided by a block 13 (PWM) and an error signal provided by an integrated error amplifier 14. VIPER circuit 11 includes an input terminal Vdd for receiving a positive power supply, a voltage reference terminal Vss, a terminal OSC determining the oscillation frequency of block 13, and a compensation terminal VVcomp of the feedback loop. Amplifier 14 is associated with an integrated voltage reference (symbolized by a zener diode DZ'), and its comparison input is internally connected to positive supply terminal Vdd. Circuit 11 also includes, of course, accesses to the two power terminals of switch 1', one of these terminals (the drain of the MOS transistor forming switch 1') corresponding to voltage reference terminal Vss.
A VIPER circuit such as illustrated in FIG. 3 characterizes by the fact that it is current-controlled and that the control, that is, the control pulse-width modulation of switch 1', is performed by varying a compensation loop of integrated circuit 11 which tends by itself to maintain its supply voltage (Vdd-Vss).
Thus, in an application to a switch-mode converter, terminal Vdd is connected, via a resistor Rp, to terminal K of application of the rectified voltage. Terminal OSC is connected to the midpoint 4 of a series association of a resistor R2 with a capacitor C2 determining, as in the case of FIG. 1, the oscillation frequency. A resistor R3, in series with a capacitor C3, is connected between terminal Vcomp and midpoint 3 of the series association of switch 1' with inductance Li (not shown). A capacitor C1 is connected between terminal Vdd forming a positive local supply line PL of circuit 11 and terminal 3 forming a negative supply line NL of this circuit, connected to terminal Vss. The use of a circuit 11 requires a zener diode DZ1 to make the local supply voltage accurate. Indeed, such an integrated circuit is provided for regulating its own supply voltage to a given value (for example, 13 volts) while being supplied under a value which can be different (for example, between 8 and 16 volts).
To vary the width of the turn-on pulses of switch 1', the potential applied to terminal Vcomp is modified, that is, the compensation loop is disturbed so that error amplifier 14 introduces a correction. This is done, for example, via a transistor 10, the collector of which is connected to terminal Vcomp and the emitter of which is connected to line NL. When transistor 10 is turned on, it modifies the supply of circuit 11 and, accordingly, the width of the turn-on pulses of switch 1'.
As previously, for an assembly such as illustrated in FIG. 3 to be able to operate in a non-isolated converter, transistor 10 must be a phototransistor of an optocoupler 9, the photodiode of which (not shown) is controlled similarly to one of the two assemblies illustrated by FIGS. 1 and 2.
Thus, even with an integrated circuit of the type shown in FIG. 3, a converter, be it a step-down/step-up converter or a step-down converter, suffers from the same disadvantages as previously. Additionally, when operating in the step-down mode, the converter cannot provide a voltage lower than the supply voltage of the integrated circuit.